312 



LOWER INVERTEBRATES. 



a friable condition, an exposure of but a single hour to moisture restored their former 

 form and elasticity, and the egg developed in the normal manner. The writer, in his 

 studies of the development of Litnax, did not have such results. The eggs, after 

 drying, were readily swollen by a moist atmosphere ; but if the desiccation had been 

 too long continued (even witliout heat), the eggs failed to develop farther. 



Like most of the shelled pulmonates, the Helicidae in temperate climates foi-m an 

 epiphragm to close the shell dui-ing the winter hibernation, and in the hotter portions 

 of the globe during the dry season. The method of forming this has thus been de- 

 scribed. " The animal being withdrawn into the shell, the collar is brought to a level 

 with the aperture, and a quantity of mucus is poured out from it and covers it. A 

 small quantity of air is then emitted from the respiratory foramen, which detaches the 

 mucus from the collar, and projects it in a convex form like a bubble. At the same 

 time the animal retreats farther into the shell, leaving a vacuum between itself and 

 the membrane, which is consequently pressed back by the external au" to a level with 

 the aperture, or even farther, so as to form a concave surface, where, having become 

 desiccated and hard, it remains fixed. These operations are nearly simultaneous, and 

 occupy but an instant. As the weather becomes colder, the animal retires farther into 

 the shell and makes another septum, and so on, until sometimes there are as many as 

 six of these partitions ; the circulation becomes slow ; the pulsations of the heart, ^hich 

 in the season of activity vary from forty to sixty in a minute, according to the tem- 

 perature of the air, decrease in frequency and strength, until they at length become 

 imperceptible ; the other functions of the body cease, and a state of torpidity succeeds, 

 which is interrupted only by the heat of the next spring's sun." With the snails which 

 occupy a constantly warm, moist climate like that of Florida, there is no period of 

 hibernation ; they are active throughout the year. 



First in order comes the Vitrininte, of which the genus Vitrina is the type. Here 

 the thin spiral shell is too small to contain the entire animal, and is 

 composed of a few rapidly enlarging whorls. The species are very 

 active and live in moist situations, usually feeding on vegetable sub- 

 stances, but not in all cases being averse to an animal diet. Three 

 species of this genus are found in the United States, while there are 

 about a hundred in the entire world. The most common form in our 

 territory is that figured, V. peUucida. 



In the next sub-family, the Zonitinae, but two genera need our 

 attention. In the genus Macrocyclis, of which only one species is 

 found east of the Rocky Mountains, the thin shell has a wide um- 

 bilicus and a sharp outer lip. M. concava is comparatively common 

 and leads an active life. It is very voracious, and feeds upon other 

 species of the family. Its body is narrow and 

 very extensible, and it thrusts it into the shell of Fig. set.— Macroq/- 



, . -I p -I IP . ^^^^ concava. 



Other species and feeds on the soft parts at its 

 leisure. Zonites contains many more species than the genus just 

 mentioned, in which the shell is much like that in Macrocyclis., the 

 differences being found almost entirely in the dentition and in the 

 soft parts. Z. cellaria is an European species which has been in- 

 troduced into Americn, where it is now common in the seaport 



towns along the Atlantic coasts. It lives in cellars and in hothouses and gardens. 



The way in which it has been introduced is uncertain. From its habits it would 



(5 



Fig. 363. —Ki^Hna 



pellticida. 



Fig. 365. — Z<mites cel- 

 laria, cellar snail. 



